Slide with synchronizing cable drive

ABSTRACT

A drawer slide structure in which two mating slide rails oppositely enclose an intermediate element which carries a roller registering with the transversely-central regions of the rails, a cable being anchored to one end of one rail, extending around the roller and being tautly anchored to the corresponding end of the other rail, and another cable being anchored to the opposite end of the other rail, extending around a roller and being tautly anchored to the opposite end of the one rail, all to provide an affirmative driving connection among the parts such that, if one rail is held stationary and the other rail is moved longitudinally in either direction, the intermediate element will be moved in the same direction and at one-half the velocity of the moving rail. In one form of the invention, there may be a single roller for both cables, in another form there may be two rollers on a common axis, but in the preferred form there are separate rollers on separate axes, each axis being spaced as far as possible from its cable anchorage when the structure is fully collapsed.

[ Aug. 29, 1972 SLIDE WITH SYNCHRONIZING CABLE DRIVE Inventors: Maxwell S. Fall; Ronald D. Lambert, both of Indianapolis, Ind.

Assignee: Herbert S. Fall, Indianapolis, Ind.

Filed: June 11, 1971 Appl. No.2 152,250

U.S. Cl. ..308/3.8, 312/110, 312/331 Int. Cl. ..F16c 29/00 Field of Search ..308/3.8; 312/110, 331, 340,

References Cited UNITED STATES PATENTS Primary ExaminerMartin P. Schwadron Assistant Examiner-Barry Grossman Att0rneyI-Iood, Gust, Irish, Lundy & Coffey [57] ABSTRACT A drawer slide structure in which two mating slide rails oppositely enclose an intermediate element which carries a roller registering with the transversely-central regions of the rails, a cable being anchored to one end of one rail, extending around the roller and being tautly anchored to the corresponding end of the other rail, and another cable being anchored to the opposite end of the other rail, extending around a roller and being tautly anchored to the opposite end of the one rail, all to provide an affirmative driving connection among the parts such that, if one rail is held stationary and the other rail is moved longitudinally in either direction, the intermediate element will be moved in the same direction and at one-half the velocity of the moving rail. In one form of the invention, there may be a single roller for both cables, in another form there may be two rollers on a common axis, but in the preferred form there are separate rollers on separate axes, each axis being spaced as far as possible from its cable anchorage when the structure is fully collapsed.

14 Claims, 7 Drawing Figures PATENTEmuczelslz 3.687305 sum 1 or 2 INVENTORS MAXWELL 5 FALL 3 A RONALD n. LAMBERT ATTORNEYS SLIDE WITH SYNCI-IRONIZING CABLE DRIVE This application relates to an improvement upon the invention disclosed and claimed in the concurrentlyfiled application of Herbert S. Fall, Willian D. York and Larry D. Huff, Ser. No. 152,258 for Ball Bearing Slide with Synchronizing Mechanism.

The present invention is directed primarily to the provision of heavy-duty means for supporting a structure during movement of that structure between what may be referred to as a rest position, and one or more extended positions, which extended positions may be oppositely spaced from the rest position of the structure. A primary object of the invention is to provide, in such a mechanism, inexpensive, readily applied, longer-lived means which is less subject to attrition for ensuring the affirmative maintenance of a balanced condition among the several parts of the slide structure.

Further objects of the invention will appear as the description proceeds.

To the accomplishment of the above and related objects, our invention may be embodied in the forms illustrated in the accompanying drawings, attention being called to the fact, however, that the drawings are illustrative only, and that change may be made in the specific constructions illustrated and described, so long as the scope of the appended claims is not violated.

In the drawings:

FIG. 1 is a fragmental, isometric view of one end of a slide structure constructed in accordance with the present invention, showing the structure in partially extended condition and with parts broken away for clarity of illustration;

FIG. 2 is a somewhat diagrammatic illustration of the two rails, the intermediate element and the flexible strand means of the present invention, illustrating the affirmative connection among the three major elements of the system;

FIG. 3 is an enlarged, transverse section taken substantially on the line 33 of FIG. 4;

FIG. 4 is an enlarged, fragmentary elevational view of a mid-portion of the assembly, parts being broken away for clarity of illustration;

FIG. 5 is a laterally-exploded end elevation of a slide structure closely similar in many ways to that of FIGS. 1 to 3 but embodying a modified form of synchronizing means;

FIG. 6 is a similarly-exploded, diagrammatic top plan view showing the modified relationship of the rails, the intermediate member, the rollers, the cables and cooperating stop means; and

FIG. 7 is an end elevation of a modified slide structure with which may be combined either the synchronizing mechanism of FIG. 6 or that of FIG. 2.

Referring more particularly to FIGS. 1-4, it will be seen that we have illustrated a fragment of a threemember slide rail structure. When such a structure is used at each side of a drawer member to support that drawer member for outward and inward movement relative to a frame, the rail 10 may be suitably secured to one side of a drawer opening in a frame while the second rail 11 is secured to one side of the drawer structure to support the same, an intermediate member 12 being interposed, and providing an operative connection, between the first and second rails 10 and 11. The intermediate member 12 is suitably retained for longitudinal reciprocation only relative to the rail 10,

and the rail 11 is suitably retained for longitudinal sliding movement only with respect to the intermediate member 12. Slide structures of this general character, as thus far described, are well known in the art to which the present invention pertains; and our invention resides in the details of structure to be described, whereby the slide structure is improved, its manufacture, assembly and maintenance are rendered more economical and maintenance of a balanced condition among the three major parts of the slide structure is affirmatively established.

As will be perceived from a careful examination of the drawings, the rail 10 comprises a first elongated channel element 13 of C-shaped cross section and a second elongated channel element 14 of C-shaped section arranged in transversely-spaced parallelism with the channel element 13 to define a parallel space 15 therebetween. The channel elements 13 and 14 are spanned and joined by a backplate 16 which may be spot welded to the backs of the elements 13 and 14 and is preferably longitudinally coextensive therewith. A stop bar 17 blocks one end of the space 15 and a similar stop bar (not shown) blocks the opposite end thereof.

The longitudinal edges of the channel element 13 are shaped to provide a curled lip 18 defining a longitudinal trackway l9 and an oppositely curled lip 20 defining a trackway 21 facing the trackway 19. Similarly, the channel member 14 has its edges shaped to provide a curled lip 22 defining a trackway 23 and a curled lip 24 defining a facing trackway 25.

A generally U-shaped ballbearing cage 26 of known construction carries a series of balls 27 at one side retentively enterable in the trackway 19 and carries a series of balls 28 at its opposite side retentively receivable in the trackway 21, whereby said cage is retained in the channel 13 for longitudinal reciprocation only relative thereto. A similar cage 29 provided with a series of balls 30 and a series of balls 31 is similarly retained for reciprocation only in the channel element 14 with its balls 30 in the trackway 23 and its balls 31 in the trackway 25 The rail 1 1 is identical, but allochirally arranged with respect to the rail 10. As is most clearly to be seen in FIG. 3, the rail 11 comprises a C-shaped channel 33 and a C-shaped channel 34 arranged in parallelism to define a parallel space 35 therebetween. A back plate 36 spans and is preferably spot welded to the back surfaces of the channel members 33 and 34 and a stop bar 37 blocks one end of that space while a similar stop bar (not shown) blocks the other end thereof. The channel member 33 is formed to provide a curled lip 38 defining a trackway 39 and a curled lip 40 defining a trackway 41 facing the trackway 39. The channel member 34 is formed to provide a curled lip 42 defining a trackway 43 and a curled lip 44 defining a trackway 45 facing the trackway 43. A cage 46 having a series of balls 47 retentively received in the trackway 39 and a series of balls 48 retentively received in the trackway 41 is thus retained for longitudinal reciprocation only in the channel member 33; while a similar cage 49 is similarly retained in the channel member 34 by engagement of its balls 50 in the trackway 43 and engagement of its balls 51 in the trackway 45.

The intermediate member 12 consists of four identical bars 52, 53, 54 and 55. The bars 52 and 53 are secured together back to back to form a leg and the bars 54 and 55 are secured together back to back to form a similar leg; and a bridge piece 56, interposed between the bar pairs, spans and joins the legs substan tially at their centers of length. As will appear more fully hereinafter, the lateral edges of the bridge piece may be substantially coincident in length with the legs of the intermediate member 12, or separate spacer pieces (not shown) may be interposed between the bars of each leg to prevent bar warpage.

Each of bars 52, 53, 54 and 55 is formed to provide, at its lateral edges, parallel, longitudinal lips 57 and 58 shaped to define outwardly-presented trackways 59 and 60 so located, when the parts are assembled, as retentively to receive the ball series 27 and 28, respectively, or the corresponding series of the associated cage. It will be perceived that, with the blocks 17 and 37 and their analogues removed, the intermediate member 12 can be telescoped into both of the rails and 11, thus engaging the balls of the several cages, whereby the assembly of three major parts will be completed.

Quite obviously, when the parts as thus far described are so assembled, and the rail 11 is moved toward the left, for instance, while the rail 10 is held stationary, the intermediate member 12 may or may not be entrained to an indeterminate extent; and when the rail 11 reaches full registry with the rail 10, the intermediate member 12 may have trailed behind the rail 11 and thus be left protruding from the right-hand end of the assembly, or it may have preceded the rail 11 and so be left protruding from the left-hand end of the assembly. Such conditions are patently undesirable, and further it is desirable that the three major elements of the assembly remain in balanced condition at all times during the complete path of movement of the drawer or other structure supported by the slide assembly. It is thus a major objective of the present invention to provide novel, inexpensive, highly effective and affirmative means to enforce the desired relative movement among the three major elements of the assembly.

To that end, we mount upon the intermediate member 12 a roller or rotor means 66 so located as to register with the spaces 15 and 35 of the respective rails 10 and 11; and we tether the two rails to each other by means of two cables, each of which is trained around the roller means 66 in a manner later to be described in greater detail. The roller is so mounted as to travel with the intermediate element 12 and one advantageous arrangement is to mount the roller upon the finger 65 of the bridge piece 56 in the manner fully disclosed in the said copending application Ser. No. 152,258.

In one form of our invention, the roller 66 is a single element formed with two axially-spaced, peripheral grooves 67 and 68. It will be apparent, however, that we could use two separate rollers, joumalled on a common axis-or upon separated axes, each being formed with one of the grooves 67 or 68.

A cable 69 or other suitable flexible strand means may preferably be formed with a bight 70 at each end thereof. One end of said cable is anchored adjacent one end of the rail 10 by any suitable means such as, for instance, one of the screws or rivets 72 which secures the block 17 and passes through the bight at that end of the cable 69. The cable is then trained about the roller 66,

being disposed in the groove 68 for guidance, and its opposite end 73 is tautly anchored to the rail 11, for instance by passing one of the screws or rivets 74 through the bight of the cable end 73.

Similarly, a second cable 75 has one end 76 anchored near the opposite end of the rail 10, is then trained about the roller 66, being disposed in the groove 67, and its other end 77 is anchored near the corresponding end of the rail 1 1.

Referring to FIG. 2 and assuming that the rail 10 is fixed to a drawer frame or the like, while one side of the drawer is carried by the rail 11, it will be seen that, if the rail 1 1 is moved toward the right relative to the rail 10, the lower run of the cable 75 will be elongated while the upper run of that cable will be shortened, whereby the roller 66 will be rotated in a counterclockwise direction and the roller, and consequently the intermediate element 12, will be bodily moved toward the right at one-half the velocity of movement of the rail 11. At the same time, of course, the lower run of the cable 69 will be shortened while the upper run thereof will be correspondingly lengthened.

Quite apparently, if the movement of the rail 11 is reversed, the intermediate member will be moved to the left at one-half the velocity of the moving rail 1 1. In the first instance, the cable 75 is the active driver for the intermediate element 12 while the cable 69 acts to restrain the intermediate element against overtravel; and in the second instance, the cable 69 is the active element and the cable 75 is the restrainer.

It will also be apparent that, if the assembly is used in a table structure wherein the intermediate element 12 is fixed to a table frame, the rail 10 carries a leaf at one end of the table and the rail 11 carries a leaf at the opposite end of the table, forcible movement of one leaf and its rail in one direction will cause a responsive movement of the other leaf in the opposite direction and to the same extent.

Because the cables 69 and 75 are of constant length, those cables will remain always taut and a balanced relationship among the three major elements of the assembly will be continuously maintained.

While the structure just described has been found to be a significant improvement over previously known slide structures, we presently believe that the form of our invention illustrated in FIGS. 5 and 6 is preferred in that it provides for an increased degree of extension in any given size of assembly, is more easily assembled and includes certain safety features over the structure illustrated in FIGS. 1 to 4.

The rail 80 is very much like the rail 10, the second rail 81 is very much like the rail 11 and the intermediate element 82 is very much like the intermediate element 12 of Flg. l. The rail 80 includes a channel or guideway 13 and a parallel channel 14 joined by a backplate 1.6. The channel 13 is formed with curled lips 18 and 20 defining trackways 19 and 21; and the channel 14 is formed to provide curled lips 22 and 24 defining trackways 23 and 25. The series of ball bearings 27 and 28 are retentively received in the trackways 19 and 21, respectively and the balls 30 and 31 are retentively received in the trackways 23 and 25, respectively.

The rail 81 comprises channels 33 and 34 joined by a backplate 36. The channel 33 is formed with curled lips 38 and 40 defining trackways 39 and 41, and the channel 34 is formed with curled lips 42 and 44 defining trackways 43 and 45. Balls 47 are retentively received in the trackway 39, balls 48 are retentively received in the trackway 41, balls 50 are retentively received in the trackway 43 and balls 51 are retentively received in the trackway 45.

The intermediate element 82 is made up of bars 52, 53 arranged back to back and bars 54, 55 arranged back to back, said bars being secured to a bridge piece similar to the bridge piece 56 but formed to provide two longitudinally-separated axles 83 and 84 upon which are respectively journalled peripherally grooved rollers 85 and 86.

Each of the bars 52, 53, 54 and 55 is formed to provide at its opposite edge longitudinal lips 57 and 58 defining trackways 59 and 60 for retentively receiving the series of balls, for instance 27 and 28, with which that bar is associated.

A first cable 87 has one end anchored near the end 88 of the rail 80, is trained about that roller 85 which is journalled on the axle 83 which is more remote from the rail end 88 when the intermediate element is in its median position, and has its opposite end tautly anchored adjacent the corresponding end 89 of the rail 81. Similarly, a cable 90, or other equivalent flexible strand means, has one end anchored near the end 91 of the rail 80, is trained around the roller 86 and has its opposite end tautly anchored adjacent the end 92 of the rail 81.

It will be perceived that, since the cable 87 is longer than it would be if the roller 85 were journalled substantially at the center of length of the intermediate element 82, and since the cable 90 is longer than it would be if the roller 86 were journalled substantially at the center of length of the said intermediate element, the total extension of the assembly of FIG. 6 is significantly increased.

In order to avoid separation of either of the rails 80 and 81 from the intermediate element 82, even in case of breakage of one of the cables 87 or 90, we provide a series of cooperating stop members on the rails and on the intermediate element. Thus, a stop 93 is located at the end 89 of the rail 81 and in interfering relationship to a stop 94 fixed to the intermediate element 82. Correspondingly, a stop 95 is fixed to the intermediate element 82 in interfering relationship to a stop 96 secured to the end 91 or the rail 80. When the parts are in fullycollapsed positions, the longitudinal distance between the stops 93 and 94 is the same as the longitudinal distance between the stops 95 and 96; and, as the rail 81 is moved upwardly as viewed in FIG. 6 while the rail 80 is held stationary, the intermediate element 82 will move in the same direction, but at half the velocity of the rail 81 so that, as the stop 95 approaches the stop 96 will engage 96, the stop 93 will approach the stop 94 at the same rate. Thus, the stop 95 will engage the stop 96 simultaneously as the stop 93 engages the stop 94.

Equivalently, a stop 97 is fixed to the end 92 of the rail 81 in interfering relationship with a stop 98 secured to the intermediate element 82, but in non-interfering interfering relationship with the stop 94. A stop 99 is secured to the intermediate element 82 in interfering relationship with the stop 100 secured to the end 88 of the rail 80, but in non-interfering relationship to the stop 96. Thus, if the rail 80 is held stationary while the rail 81 is moved downwardly as viewed in Flg. 6, the stop 99 will approach the stop 100 at the same rate at which the stop 97 approaches the stop 98 and stop 99 will engage stop 100 simultaneously as the stop 97 engages the stop 98.

In FIG. 7, we have shown a modified form of ball bearing slide rail assembly. In this form, a first rail, indicated generally by the reference numeral 110, comprises a channel 113 and a channel 114 secured together in parallelism by a backplate 116. A second rail, indicated generally by the reference numeral 111, comprises a bar 203, a parallel bar 205 and a backplate 166 spanning and joining said bars. An intermediate element 1 13 comprises a bar 152 secured back-to-back to a channel 163, a bar 154 secured back-to-back to a channel 164 and a bridge piece 156 interposed between, spanning and secured to, the bar and channel pair 152, 163 and the bar and channel pair 129, 164. A ball bearing cage 126 with ball series 127 and 128 retentively connects bar 152 to the channel 113 for longitudinal movement only relative thereto and a ball bearing cage 129 with ball series 130 and 131 retentively connects the bar 154 to the channel 114 for longitudinal movement only relative thereto.

Similarly, a ball bearing cage 176 with balls 177 and 178 and a ball bearing cage 179 with ball bearings 180 and 181 retentively connect the rail 111 to the intermediate element 113 for longitudinal movement only relative thereto.

As shown, rollers 85 and 86 are mounted on the bridge piece 156 in the relationship discussed in describing FIGS. 5 and 6, and cables 87 and 90 provide the same kind of driving connection among the three major elements of the slide structure.

It will be apparent that the slide structure of FIG. 7 provides stronger resistance to the moment impressed upon the slide structure when the structure is extended than is provided by the structure of FIG. 1 or FIG. 5. It will also be apparent that additional units may be readily added to the assembly of FIG. 7 to provide for still greater extension. That is, if the solid backplate 166 is replaced by a bridge piece like the element 156, a pair of channels like the channels 163 and 164 may be welded to that additional bridge piece and an additional pair of bars like the bars 203 and 205, with their associated ball bearings cages can be retentively engaged in the additional channels.

While several preferred embodiments of our invention have been illustrated and described in detail, it will be apparent that many of its advantageous results can be achieved by different means. For instance, it would be possible to eliminate the roller means completely by simply rounding and smoothing the mutually-facing edges of the legs62 and 63, as shown in FIG. 4, training the cable 69 about the leg 62 and the cable about the leg 63 and permitting the cables to slide around the rounded leg edges. Or separate roller means, on separated axes, might be provided with the respective cables 69 and 75 trained about the respective rollers nearer their anchorages. Alternatively, the roller means might be sprockets and the cables might be replaced by chains, meshing with the sprockets. The essential requirement is that there shall be two flexible strands, each trained about abutment means carried by the intermediate element, and providing a driving connection among the three major elements such that, when one rail is moved in either direction while the other is held stationary, the intermediate element will be driven in the same direction but at one-half the velocity.

We claim:

In combination, a first elongated member providing longitudinal guide means, a second elongated member providing corresponding longitudinal guide means, an intermediate element providing slide means retentively associated with both guide means for relative longitudinal movement only, means moving with said intermediate element and establishing an abutment surface facing away from one end of said members and an abutment surface facing away from the opposite end of said members, flexible strand means anchored near one end of said first member, trained about the abutment surface which faces away from said one end of said first member "and tautly anchored near the corresponding end of said second member, and flexible strand means anchored near the opposite end of said second member, trained about the abutment surface which faces away from said opposite end of said second member and tautly anchored near the opposite end of said first member.

2. The combination of claim 1 in which said means establishing said abutment surfaces is roller means.

3. The combination of claim 2 in which said roller means is journalled on a single axis fixed relative to said intermediate element and extending transversely relative to the length of said intermediate element.

4. The combination of claim 3 in which said roller means is a single roller formed with separate peripheral grooves for the reception of said strand means, respectively.

5. A device of the class described comprising a first rail providing two transversely-spaced, longitudinally parallel channels, a second rail providing two transversely-spaced longitudinally parallel channels, said channels of said second rail being arranged in facing registry with the channels of said first rail, an intermediate element having parallel legs slidably received, respectively, in the channels of both rails, a bridge member joining said legs substantially at their centers of length, roller means journalled on a transverse axis on said bridge member with its periphery aligned with the spaces between the channels of said rails, an elongated ball bearing cage retained in each of said channels for longitudinal reciprocation therein, each cage further having retaining engagement with one of said legs, flexible strand means anchored near one end of said first rail, trained about said roller means and tautly anchored near the corresponding end of said second rail, and flexible strand means anchored near the opposite end of said second rail, trained about said roller means and tautly anchored near the corresponding end of said first rail.

6. The device of claim 5 in which said anchorages are all located in registry with the spaces between the channels of said rails.

7. The device of claim 5 in which said roller means is a single roller and said strand means are trained in a common direction about said roller.

8. The device of claim 7 in which said roller is formed with separate peripheral grooves for the reception of said strand means, respectively.

9. In combination, a first elongated member providing longitudinal guide means, a second elongated member providing corresponding longitudinal guide means, an intermediate element providing slide means retentively associated with both guide means for relative longitudinal movement only, a roller journalled on a transverse axis on said intermediate element, and flexible strand means anchored to said rails adjacent opposite ends thereof and tautly trained about said roller to establish a driving connection whereby, when one rail is held stationary and the other is moved in either direction longitudinally, the intermediate element will be moved in the same direction and at onehalf the velocity.

10. A device of the class described comprising a first rail providing two transversely-spaced, longitudinally parallel channels, a second rail providing two transversely-spaced longitudinally parallel channels, said channels of said second rail being arranged in facing registry with the channels of said first rail, an intermediate element having parallel legs slidably received, respectively, in the channels of both rails, a bridge member joining said legs substantially at their centers of length, a roller journalled on a transverse axis on said bridge member with its periphery aligned with the spaces between the channels of said rails, an elongated ball bearing cage retained in each of said channels for longitudinal reciprocation therein, each cage further having retaining engagement with one of said legs, and flexible strand means anchored to said rails adjacent opposite ends thereof and tautly trained about said roller to establish a driving connection whereby when one rail is held stationary and the other is moved in either direction longitudinally, the intermediate element will be moved in the same direction and at onehalf the velocity.

11. In combination, a first elongated rail, a second elongated rail, an intermediate elongated member having retentive engagement with both of said rails for relative longitudinal movement only, abutment means carried by said intermediate member and facing away from one end of said rails, flexible strand means anchored to said one end of said first rail, trained about said abutment means and tautly anchored to the corresponding end of said second rail, abutment means carried by said intermediate member and facing away from the other end of said rails, and flexible strand means anchored to the other end of said first rail, trained about said last-named abutment means and tautly anchored to the other end of said second rail.

12. The combination of claim 11 in which each abutment means is a roller mounted upon an axis disposed in the plane of travel of said intermediate member and perpendicular to the line of travel thereof.

13. The combination of claim 12 in which the axis of said first roller is disposed adjacent that end of said intermediate member which is remote from said one end of said rails when said member is in its median position, and said second roller is disposed adjacent the other end of said intermediate member.

14. The combination of claim 13 including cooperating stop elements on said rails and on said intermediate member to limit relative travel thereof to prevent any strand anchorage from reaching the axis of the roller about which it is trained.

1737 UNITED STATES PATENT OFFICE CERTEFECATE 0F CGRECTIQN Patent 3,687,505 Dated August 29', 1972 fl Maxwell S. Fall and Ronald D. Lambert It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 5, line 15, "edge" should be edges line 49, "or" should be of line 57, delete "will engage 96"; line 63, delete "interfering".

Column 6, line 48, "bearings" should be bearing Column 7, line 6 (Claim 1, line 1) insert l.

at the beginning of the line, before "In".

Signed and sealed this 23rd day of January 1973..

(SEAL) Attest:

ROBERT GOTTSCHALK Commissioner of Patents EDWARD M.PLETCHER,JR. Attesting Officer 

1. In combination, a first elongated member providing longitudinal guide means, a second elongated member providing corresponding longitudinal guide means, an intermediate element providing slide means retentively associated with both guide means for relative longitudinal movement only, means moving with said intermediate element and establishing an abutment surface facing away from one end of said members and an abutment surface facing away from the opposite end of said members, flexible strand means anchored near one end of said first member, trained about the abutment surface which faces away from said one end of said first member and tautly anchored near the corresponding end of said second member, and flexible strand means anchored near the opposite end of said second member, trained about the abutment surface which faces away from said opposite end of said second member and tautly anchored near the opposite end of said first member.
 2. The combination of claim 1 in which said means establishing said abutment surfaces is roller means.
 3. The combination of claim 2 in which said roller means is journalled on a single axis fixed relative to said intermediate element and extending transversely relative to the length of said intermediate element.
 4. The combination of claim 3 in which said roller means is a single roller formed with separate peripheral grooves for the reception of said strand means, respectively.
 5. A device of the class described comprising a first rail providing two transversely-spaced, longitudinally parallel channels, a second rail providing two transversely-spaced longitudinally parallel channels, said channels of said second rail being arranged in facing registry with the channels of saiD first rail, an intermediate element having parallel legs slidably received, respectively, in the channels of both rails, a bridge member joining said legs substantially at their centers of length, roller means journalled on a transverse axis on said bridge member with its periphery aligned with the spaces between the channels of said rails, an elongated ball bearing cage retained in each of said channels for longitudinal reciprocation therein, each cage further having retaining engagement with one of said legs, flexible strand means anchored near one end of said first rail, trained about said roller means and tautly anchored near the corresponding end of said second rail, and flexible strand means anchored near the opposite end of said second rail, trained about said roller means and tautly anchored near the corresponding end of said first rail.
 6. The device of claim 5 in which said anchorages are all located in registry with the spaces between the channels of said rails.
 7. The device of claim 5 in which said roller means is a single roller and said strand means are trained in a common direction about said roller.
 8. The device of claim 7 in which said roller is formed with separate peripheral grooves for the reception of said strand means, respectively.
 9. In combination, a first elongated member providing longitudinal guide means, a second elongated member providing corresponding longitudinal guide means, an intermediate element providing slide means retentively associated with both guide means for relative longitudinal movement only, a roller journalled on a transverse axis on said intermediate element, and flexible strand means anchored to said rails adjacent opposite ends thereof and tautly trained about said roller to establish a driving connection whereby, when one rail is held stationary and the other is moved in either direction longitudinally, the intermediate element will be moved in the same direction and at one-half the velocity.
 10. A device of the class described comprising a first rail providing two transversely-spaced, longitudinally parallel channels, a second rail providing two transversely-spaced longitudinally parallel channels, said channels of said second rail being arranged in facing registry with the channels of said first rail, an intermediate element having parallel legs slidably received, respectively, in the channels of both rails, a bridge member joining said legs substantially at their centers of length, a roller journalled on a transverse axis on said bridge member with its periphery aligned with the spaces between the channels of said rails, an elongated ball bearing cage retained in each of said channels for longitudinal reciprocation therein, each cage further having retaining engagement with one of said legs, and flexible strand means anchored to said rails adjacent opposite ends thereof and tautly trained about said roller to establish a driving connection whereby when one rail is held stationary and the other is moved in either direction longitudinally, the intermediate element will be moved in the same direction and at one-half the velocity.
 11. In combination, a first elongated rail, a second elongated rail, an intermediate elongated member having retentive engagement with both of said rails for relative longitudinal movement only, abutment means carried by said intermediate member and facing away from one end of said rails, flexible strand means anchored to said one end of said first rail, trained about said abutment means and tautly anchored to the corresponding end of said second rail, abutment means carried by said intermediate member and facing away from the other end of said rails, and flexible strand means anchored to the other end of said first rail, trained about said last-named abutment means and tautly anchored to the other end of said second rail.
 12. The combination of claim 11 in which each abutment means is a roller mounted upon an axis disposed in the plane of travel of said intermediate member and perPendicular to the line of travel thereof.
 13. The combination of claim 12 in which the axis of said first roller is disposed adjacent that end of said intermediate member which is remote from said one end of said rails when said member is in its median position, and said second roller is disposed adjacent the other end of said intermediate member.
 14. The combination of claim 13 including cooperating stop elements on said rails and on said intermediate member to limit relative travel thereof to prevent any strand anchorage from reaching the axis of the roller about which it is trained. 